SAM-M10Q - Integration manual
enabled, SU if GLONASS is enabled, NTSC if BeiDou is enabled and, finally, European if Galileo is
enabled.
The receiver will assume that an input time pulse uses the same GNSS time base as specified for the
time pulse output. So if the user selects GLONASS time for time pulse output, any time pulse input
must also be aligned to GLONASS time (or to the separately chosen variant of UTC). Where UTC is
selected for time pulse output, any GNSS time pulse input will be assumed to be aligned to GPS time.
The receiver allows users to independently choose GNSS signals used in the receiver (using
CFG-SIGNAL-*) and the input/output time base (usingCFG-TP-*). For example it is possible
to instruct the receiver to use GPS and GLONASS satellite signals to generate BeiDou time.
This practice will compromise time pulse accuracy if the receiver cannot measure the timing
difference between the constellations directly and is therefore not recommended.
The information that allows GNSS times to be converted to the associated UTC times is
only transmitted by the GNSS at relatively infrequent periods. For example GPS transmits
UTC(USNO) information only once every 12.5 minutes. Therefore, if a time pulse is
configured to use a variant of UTC time, after a cold start, substantial delays before the
receiver has sufficient information to start outputting the time pulse can be expected.
Each GNSS has its own time reference for which detailed and reliable information is provided in the
messages listed in the table below.
Time reference
Message
GPS time
UBX-NAV-TIMEGPS
BeiDou time
UBX-NAV-TIMEBDS
GLONASS time
UBX-NAV-TIMEGLO
Galileo time
UBX-NAV-TIMEGAL
QZSS time
UBX-NAV-TIMEQZSS
UTC time
UBX-NAV-TIMEUTC
Table 13: GNSS time messages
2.7.3 Navigation epochs
Each navigation solution is triggered by the tick of the 1-kHz clock nearest to the desired navigation
solution time. This tick is referred to as a navigation epoch. If the navigation solution attempt is
successful, one of the results is an accurate measurement of time in the time base of the chosen
GNSS system, called GNSS system time. The difference between the calculated GNSS system time
and receiver local time is called the clock bias (and the clock drift is the rate at which this bias is
changing).
In practice the receiver's local oscillator will not be as stable as the atomic clocks to which GNSS
systems are referenced and consequently clock bias will tend to accumulate. However, when
selecting the next navigation epoch, the receiver will always try to use the 1-kHz clock tick which it
estimates to be closest to the desired fix period as measured in GNSS system time. Consequently
the number of 1-kHz clock ticks between fixes will occasionally vary. This means that when
producing one fix per second, there will normally be 1000 clock ticks between fixes, but sometimes,
to correct drift away from GNSS system time, there will be 999 or 1001.
The GNSS system time calculated in the navigation solution is always converted to a time in both
the GPS and UTC time bases for output.
Clearly when the receiver has chosen to use the GPS time base for its GNSS system time, conversion
to GPS time requires no work at all, but conversion to UTC requires knowledge of the number of
leap seconds since GPS time started (and other minor correction terms). The relevant GPS-to-UTC
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2 Receiver functionality
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